Mixer circuits are used in a variety of applications. For example, they are often used in radio frequency (RF) applications for up-converting or down-converting. In this context up-converting is the process of mixing a baseband signal such as a differential baseband signal, with an RF signal that is generated by a local oscillator circuit that operates in the RF range. This process generates a mixed RF signal with the baseband information included within the RF signal generated by the local oscillator. Down converting is the process of separating the baseband signal from the mixed RF signal generated by the local oscillator. When the radio frequency (RF) spectrum is directly translated to the baseband in the first down-conversion, the receiver is called a “homodyne,” “direct-conversion,” or “Zero-IF” architecture.
FIG. 1 shows a conventional double balanced mixer 100. Double balanced mixer (DBM) 100 includes two mixer circuits the outputs of which are coupled to common load. DBM 100 is comprised generally of two sets of differential transistor pairs M1/M2 110/112 and M3/M4 114/116, each of which receives a first differential input signal RF_P, RF_M, respectively. The two sets of differential transistor pairs M1/M2 110/112 and M3/M4 114/116 receive a second differential signal LO_P, LO_M that drives the gates of transistor pairs M1/M3 110/114 and M2/M4 112/116, respectively. This second differential signal typically arrives from a local oscillator (LO). The biasing of the gate, drain and source terminals of transistors M1 110, M2 112, M3 114 and M4 116 is well known in the art and shall not be described.
While M1 110, M2 112, M3 114 and M4 116 are shown as transistor devices, it is known to substitute any equivalent amplifying or switching device in the configuration shown. The term passive is generally used to indicate that a mixer configuration performs no amplification. The term active is generally used to indicate that a mixer configuration performs amplification. Complementary-metal-oxide (CMOS) semiconductor technology is a common fabrication process technology.
It is desirable to match M1 110, M2 112, M3 114 and M4 116. However manufacturing variations may cause components of the circuit to be mismatched. Such mismatches may cause the output of the circuit to include a variety of unwanted frequencies. For example mismatch in the differential devices M1 110 and M2 112 or M3 114 and M4 116 may cause even harmonics. In some cases, harmonic impurities resulting from manufacturing variances may be very small and effectively negligible. When the variances are sufficiently large, harmonic impurities may impact system performance. For example, in a DBM for a direct conversion receiver, second order inter-modulation (IM2) products in particular may especially degrade the signal-to-noise ratio (SNR) at the baseband. Large manufacturing variations may even cause the system to be completely inoperable. Thus, some portions of the circuits produced by the manufacturing process may have to be discarded, thereby affecting the “yield” of the process. As manufacturing is moving to smaller process nodes, controlling the manufacturing process variances is increasingly difficult, thus methods to improve the spectral purity of mixer circuits have been presented.
FIG. 2 shows the double balanced mixer of FIG. 1 with IM2 product mismatch correction.
To address the effects of mismatch, one or more bias voltages of transistors M1 210, M2 212, M3 214 and M4 216 may be adjusted. In the specific example of FIG. 2, the gate terminals of transistors M2 212 and M4 216 are connected to a gate voltage Vg through resistors R2 214 and R4 218, respectively. Vg may not be configurable, as when tied to an on-chip voltage reference.
A digital-to-analog converter (DAC) 222 configures the gates of transistors M1 210 and M3 214 through resistors R1 212 and R3 216, respectively. Resistors R1 212, R2 214, R3 216 and R4 218 nominally have the same value. More specifically, when the differential signals IF_P and IF_M are not matched, DAC 222 introduces an appropriate DC voltage at the gate of the two devices M1 210 and M3 214, to cause the signals IF_P and IF_M to closely match. Capacitors C1P 202, C1M 206, C2P 204 and C2M 208 serve to couple only the AC components of the signals LO_P and LO_M to the mixer, while isolating the DC voltage of the local oscillator circuit from the bias voltage at the gate of devices M1 210, M2212, M3 214 and M4216, respectively. Capacitors C1P 202, C1M 206, C2P 204 and C2M 208 nominally have the same capacitance value.
In the configuration shown in FIG. 2, device mismatches are corrected without regard to spectral purity at the output caused by possible mismatch between AC coupling capacitors C1P 202, C1M 206, C2P 204 and C2M 208. Mismatch between AC coupling capacitors C1P 202, C1M 206, C2P 204 and C2M 208 is not generally a main source of IM2 products when large capacitors are used to couple the local oscillator signal to the mixer.
However, as devices get smaller and smaller, driven particularly by the demand for smaller and smaller multi-mode, multi-protocol radio transceiver architectures, a need arises for reducing the die area and power consumed by mixers incorporated therein.
As technologies scale down, active devices (such as diodes and transistors) incorporated therein also scale down in size. Passive devices (i.e., resistors, capacitors and inductors) by comparison do not scale down proportionately. The end result is that passive components become a significant stumbling block to miniaturization and power efficiency. This is particularly the case with AC coupling capacitors used in mixers.
The optimum size of an AC-coupling capacitor size is directly related to a desired frequency of operation of the mixer. As the desired frequency of operation increases, the value and hence the size of the AC coupling capacitor value may also be reduced.
Any reduction, however, in local oscillator AC-coupling capacitance motivated by a desire to reduce capacitor size results in greater mismatch. This mismatch will cause a coupling signal strength difference between the LO_P and LO_M signals. This in turn can impact IM2 performance of the mixer requiring calibration and correction, which may not be possible without additional circuitry contributing to additional area, circuit complexity and power inefficiency.
Minimizing this additional source of impact without any additional calibration circuitry is beneficial in double balanced mixers where one or more local oscillator (LO) capacitors for a same polarity are split for IM2 calibration purposes.